International
Carpathian Control
Conference ICCC’ 2002
MALENOVICE,
CZECH REPUBLIC
May 27-30, 2002
SCIENTIFIC RESEARCH OF AFC DRIVE SYSTEMS WITH
SILICON CONTROLLED RECTIFIER
Janusz ROSIKOWSKI
Coal Mine “Piast”, Poland
“Nadwiślańska” Stock Coal Company
43-155 Bieruń, ul. Granitowa 16
tel (+48)(32) 216 25 45
e-mail: jrosikowski@nsw.com.pl
Abstract: The drive system with silicon controlled rectifier installed in the armoured face
conveyors has been subjected to industrial investigation carried out in an average productive longwall at the Piast coal mine in order to determine and prove its special features. The
apparatus and measuring system have been described in the paper. Results of the measurements presented in a form of oscillograms have been analysed. Finding of the analysis and
conclusions are contained in the final part of the paper.
Key words: armoured face conveyor (AFC), drive system with silicon controlled rectifier
(SCR), industrial investigations
1 Introduction
At present the armoured face conveyors are the machines of higher power installed in
underground hard coal mines. In the USA and Australia maximal power of drives of the
AFC augment 2.565 MW, in Poland – 1,5 MW. It is planned to implement the drives of the
motor power from 1000 to 1500 kW supplied with 11000 V.
At present in the Piast Colliery, the biggest in Europe, of the production rate of about
30000 t/d, since 1994 intensive testing of drives and complete flight-bar conveyors, aimed
at an optimal selection of the power and capacity at ensuring a soft start of the flight-bar
chain and others, has been carried out. The tests of the drives equipped with two-speed
motors, of the purely electro-mechanical drives with a direct start-up, special types of the
CST drive systems of the DBT Company and the drives equipped with hydrodynamic,
water-emptying couplings, have been conducted.
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To soften the dynamics of the chain start-up a drive with a thyristor start has been introduced in one of the armoured face conveyors. This drive has been subjected to testing
and the results are presented in this paper.
2 Technical Equipment at the Coal Longwall Face
The measurements were taken in the longwall face 141 A/Section G1 at the Piast Colliery in March 2001 according to the strictly determined programme. The tests concerned
the armoured face conveyor and the beam stage loader, whose drives were equipped with
the soft-start systems EH-dG-A6 and EH-KK-A1. The longwall face length was 217 m, its
height varied from 1.7 to 1.9 m and the water supply was from 1 to 2 m3/min. The mechanical part of the longwall face included: Glinik 08/22 powered roof supports, KGS
345N/2BP shearer of the productivity 750 t/h, R 225/750/BP/WB/2 armoured face conveyor of the capacity 900 t/h and the power of drives 2x85/250 kW, equipped with the
central chain 2x30x108 mm of the speed 1 m/s.
The GROT-80 225/750 beam stage loader of the capacity 1100 t/h was equip-ped with
the drive with the motor of 135 kW power and the central chain 2x26x92 mm of the speed
1.31 m/s and with the Kruk-2 crusher. The supply voltage was 1000 V.
The soft-start control system consists of the analog-digital controller, the DKW
rotation direction and three push-pull thyristor modules, one for each phase. The motor
starting is realized by a method of the soft increase in the starter voltage at a simultaneous
control of the starting current. It is performed on the basis of a phase control of three pushpull thyristor modules. The controller affects the thyristors in such a way that the voltage of
each thyristor module increases gradually to the rated value in a set period of time, thus
controlling the starting current inversely proportionally to the set starting time. The
technical specification of the installed thyristor starters is as follows: supply voltage of 500
or 1000 V through an exchange of a programming plug, a possibility of sectioning the bus
bars and supplying from two transformers, a control of the protective conductor continuity,
a cooperation with any loud-speaking signalling system or automation of conveyors, a
cooperation with posistor or bimetallic sensors of the motors’ temperature. Besides, in the
starters, instead of a big number of single separators the mFk separator was used to realize
the control system in accordance with the regulations.
The mFk programmable separator has some outstanding solutions, enabling to
programme different operational regimes of the thyristor starter, resulting from the accepted
technological process. This separator offers broad possibilities of considering different
alternatives of the supplied equipment and acts as an indicator of the equipment condition.
The thyristor starters are manufactured in two types of the flame-proof enclosure:
medium-dG and small-KK.
The EH-dG-A6 starter (fig.1) in the medium enclosure is equipped with two
independent 400 A current circuits, enabling to supply the conveyor from two mobile
transformer stations and two 400 A thyristor units. Each of the two thyristor units is
equipped with two completely protected outlets and enables to supply up to two motors of
the maximal power of 530 kW at the conveyor supply voltage of 1000 V.
In the beam stage loader the thyristor starter in the small enclosure of the type EH-KKA1 (fig. 2), equipped with one 400 Amps thyristor unit and two completely protected outlets, enabling to supply up to two motors of the total power 530 kW at the voltage of 1000
V, is applied. All the outlets from the starters are equipped with the protections against:
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overload, short-circuit and load asymmetry, 1000/500 V and 42 V leakage, temperature for
a control of the motor windings and the radiators of thyristors, for the control of the earthing system continuity and overvoltage.
An innovatory, microprocessor system of thyristor release, used in these starters is
Figure 1. EH-dG-A6 thyristor starter
Figure 2. EH-KK-A1 thyristor starter
characterized by: steepness and time of increase of the pulse from 4.5 A/s and the current
pulse amplitude, exceeding by ten fold the quantity of the rated current of the thyristor gate.
Such a solution guarantees obtaining the release parameters, ensuring: maximal thyristor
resistance to the steepness in increase of the anodic current (damages at short-circuits) and
a significant decrease in heat losses, which exclude missing of the thyristor firing, thus
limiting the contents of harmonics to minimum.
These solutions increase the reliability of the system within the range from 5 to 30 s
and they ensure a correct control of the starting torque indirectly by an adjustment of the
initial voltage within the range from 5 to 50% of the rated voltage.
In the case of the thyristor starter of the EH-KK-A1 type it is possible to use
additionally a system of thyristors’ release, ensuring dynamic breaking of the motors using
direct current. When the button “switch off” is pressed, the dynamic breaking of motors
using the direct current is started; when it is finished (after opening the current contacts of
the main contactor) locking of the conveyor by the brakes and release devices is performed.
3 Testing of Thyristor Starters Supplying Flight-bar Conveyors
In March 2001 in the 141A longwall face same tests were conducted. Their objective
was a comparison, in exactly the same mining and geological conditions, of the static and
dynamic properties of the armoured face conveyor and the beam stage loader equipped with
the drives with:
− one-speed motors with a traditional start-up-stiff type,
− two-speed motors,
− one-speed motors and the soft-start.
The taken measurements enabled, among others, to carry out a detailed analysis and an
assessment of: the quantity of loads of the individual motors of the discharge and return
drives in the armoured face conveyors at an application of different conveyor starting sys537
tems, the quantity of loads of the motor in the beam stage loader at an application of different conveyor starting systems, the quantity of loads in the individual chain strands at the
start-up, using the starters under testing in comparison with a direct start-up (stiff) and
starting with the two-speed motor,etc.
In fig.3 the diagram of the measurement system is presented. A similar arrangement of
the instrumentation and sensors was used in the case of the beam stage loader.
Figure. 3. Arrangement of
measurement sensors in the
Rybnik 225/750/WB/BP
armoured face conveyor in
the 141A longwall face at
the Piast Colliery
Oscillograms of the active power consumption by the motors, pulling force and the
speed of the flight-bar chain in the armoured face conveyor at the thyristor start-up is presented in fig. 4. Obtained oscillograms from starting the armoured face conveyor and the
beam stage loader, both empty and loaded with the run-of-mine, equipped with the drives
of a different principle of operation, were a basis for an analysis of the mechanical and
electrical properties of the drives under testing.
4 Analysis of the Test Results
The conducted tests should be reckoned among the unique ones due to a differentiated
programme, the testing site, used measurement and analytical instrumentation, obtained test
results and a factographical record. The test results can be used for a further improvement
of thyristor systems of a soft start and for a further development of the theory of soft-starts
in flight-bar conveyors. It has been acknowledged that the most essential parameters for a
conveyor start-up are: maximal force in the chain, total starting time, supply voltage drop
and delays between the chain starting moments at the return point and at the discharge point. The results of the analysis for the individual types of start-ups are presented in fig. 5.
Special attention should be paid to the fact that the voltage drop during the thyristor start-up
is caused on purpose, because it is aimed at a reduction of the motor torque.
In the conveyors of low power pure electromechanical drives with a contactor start-up,
equipped with a silent block flexible coupling are used. From the tests it can be concluded
that the above system had a small influence on decreasing the dynamics of the flight-bar
conveyor loads. The start-up was characterized by a maximal current consumption by electric motors, obtaining very high acceleration of spinning masses and thus an occurrance of
high dynamic loads. A reaction of the control and supply systems to the maximal overloads
consisted in switching off the motors and interrupting the start-up. Besides, two (or more)
driving units, installed in the conveyor, were unevenly loaded. However, the drive with
two-speed motors and prolonged start-up is used most often. The relationship between the
power on both gears in most cases equals 1:3, whereas the values of torques on both gears
are similar, because two-speed motors, made for a constant torque, are in operation.
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Figure 4. Oscillogram from thyristor starting of the armoured face conveyor loaded with the
run-of-mine; 1 – active power of the discharge motor, kW; 2 – active power of the return
motor, kW; 3, 4 – A1 and A2 chain tensioning force, kN; 5, 6 – chain speed at the discharge
point and at the return point, x100m/s; 7 – total tensioning force in the chain, kN
The courses of currents, as regards the values, are different at both gears. The conveyor is started in the first gear and switching over to the second gear is performed automatically in the function of time or the value of the current at the first gear. These drives require
extended switching and control apparatus,
enabling quick switching over of the motor
from the first gear into the second one. A big
disadvantage in the case of these motors is the
rotor moment of inertia of high value.
Figure 5. Analysis results of the selected parameters, characterizing different ways of starting
the armoured face conveyor loaded with the
run-of-mine; 1 – start-up delay at the discharge
point, s; 2 – starting time, s; 3 – tensioning
force in the flight-bar chain, x10 kN; 4 – voltage drop in the percentage of the rated voltage,
% Un.
In the conveyors with this type of drive the phenomenon, lasting several dozen seconds, consisting in a graduated equalization of the load between the drives, occurs in the
post-starting period. An appropriate operation of these drives requires a correct initial tension and a proper use of the flight-bar chain.
The drive with the motor, controlled by a thyristor starter, is only in a period of the
trial operation at the Piast Colliery. The thyristor starters enable a gradual increase of the
starting torque, given by squirrel-cage motors, contributing to a reduction of disadvanta539
geous torque waves within the total start-up period. In this drive the microprocessor thyristor block controller has a task of correct shaping the conveyor start-up process through a
parametrization of: the initial voltage, the limiting current and the duration of the start-up
period. After a termination of the start-up, to limit the amount of heat released inside the
equipment, the thyristors are shunted by shunting connectors. In the case of a failure in the
thyristor system or a heavy start-up of the conveyor this driving system transforms itself
into a pure electromechanical system. The heat losses in the motor-thyristors system cause
that the drive is switched off. Due to the fact that the thyristor system of a soft-start limits
the current voltage supplying the motors, so according to the second power of the relationship between the actual voltage and the rated voltage, the actual torque of the motor decreases, and in the final phase of the start-up period the drop of the torque from the value
related to the decreased supply voltage to the value related to the rated voltage, occurs. It
can be concluded from the tests (see fig.4) that this drop does not cause an increase in the
dynamic load of the chains. The relationship between the maximal chain tensioning force
during the start-up and the average chain tensioning force, occurring in the steady motion,
varies from 1.2 to 1.3 in this type of drives. During the start-up the power of the motors
does not exceed the rated power and the start-up itself is soft and lasts about 7 s which
corresponds with the average value of the chain acceleration in the conveyor loaded with
the run-of-mine from 0.14 to 0.15 m/s2. In the case of applying this type of the soft-start,
the power of driving motors should be increased by about 20%.
This staring system is not suitable for the motors of the power exceeding 315 kW and
also for heavy and very heavy start-ups, which often occur in armoured face conveyors.
This type of drive does not ensure a proper equalization of the load between the drives and
it does not have any protection against overloading.
5 Conclusions and Final Remarks
Detailed conclusions, concerning the start-ups of flight-bar conveyors, using soft-start
systems can be formulated as follows: the system demonstrated a full operational readiness
and it limited dynamic loads in the driving system and in the conveyor chain, it caused
lower voltage drops and it ensured the starting acceleration of a small value.
Besides, the soft-start system is characterized by: the peak power at the time 0.7÷0.85
s in the last phase of the start-up of the value two times higher than the rated power value
and by the ramp of the current intensity lasting slightly above 2 s. An application of the
soft-start system requires an appropriate selection of the motors to the flight-bar conveyors
with additional about 20% of the power due to high starting resistance of these conveyors
and periodical, significant overloading with the run-of-mine.
References
ANTONIAK J.: Industrial Investigations of AFC High-Tech Drive Systems CST 30.
ICCC'2001, AGH Kraków-Krynica 22-25.05.2001, p. 153-159
LUTYŃSKI A. & ROSIKOWSKI J.: Rozruch przenośnika zgrzebłowego Rybnik 80 z użyciem
rozrusznika tyrystorowego typu OW-0142R. Materiały Sympozjum Ryfamex’98.
Rybnicka Fabryka Maszyn Ryfama, Rybnik 2-4 czerwiec 1998.
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